[Tutor] Number Coercion: I swear, I never laid a hand on it!

[Remco Gerlich]

On  0, kromag@nsacom.net wrote:
> -------------silly menu script-------------
> 
> import os
> import string
> import sys
> 
> 
> menu_items={'edit': '\windows\command\edit.com',
> 	'vim': '\vim\vim57\gvim.exe'}

Note that this won't work; backslashes have a special meaning, they're used
to escape things so you can put special characters in a string (like \n
means newline).

A way to avoid this is to escape the backslashes; use \\ instead of \.

> print 'menu choices'
> print menu_items.keys()
> 
> choice = raw_input()
> 
> if menu_items.has_key(choice):
> 	os.system('s%')% menu_items[choice]
> else:
> 	sys.exit('Sorry, try again.')
> 
> ---------end silly menu script-------------
> 
> For some reason, I get a problem with: 
> 
> 	os.system('s%')% menu_items[choice]

I don't know if this is a typo or what, but the way it is written, it runs
os.system('s%') first. 's%' is a command that probably doesn't exist, and
os.system will return some number.

The '%' operator used on numbers is the modulo operator; Python tries to
interpret menu_items[choice] as another number but fails; it's a string.
This gives a coercion error.

What you meant was both '%s' instead of 's%', and to have the % command
inside the os.system, not outside of it:

os.system('%s' % menu_items[choice])

which happens to be exactly the same thing as

os.system(menu_items[choice])


I think this reply is a bit obfuscated but I have to go, cooking :)

-- 
Remco Gerlich